1. Field of the Invention
This invention relates to a liquid-jet printing head which operates for printing by ejecting a printing liquid by utilizing thermal energy so that a flying droplet of the printing liquid is formed, and a liquid-jet printing apparatus having the liquid-jet printing head. The present invention is applied to printing of ink on various ink supporting members (recording members), such as cloth, strings, paper, and other sheet-like members, and to various information processors or to printers provided as output units of information processors.
2. Description of the Related Art
In liquid-jet printing systems, various means are used to eject a printing liquid such as ink through a head so that the ejected ink is attached to an ink supporting member such as paper, thereby performing printing.
Low-noise, high-speed and high-density printing can be achieved if a liquid-jet printing apparatus using this kind of printing system is used. Moreover, such a printing apparatus can be reduced in size and can be mass-produced easily and at a low cost, because it requires no development and fixation steps with respect to plain paper. Liquid-jet printing apparatuses have attracted attention because of these advantages.
In particular, on-demand type liquid-jet printing apparatuses do not require a high-voltage generating unit and an unnecessary-ink recovery unit, such as those required by continuous type liquid-jet printing apparatuses, and can, therefore, be reduced in overall size. It is therefore believed that they will find promising applications.
Among apparatuses of this kind, a liquid-jet printing apparatus having a liquid-jet printing head as disclosed in Japanese Patent Publication No. 61-59914, has attracted a great deal of attention. In this printing head, a liquid channel filled with a liquid is heated to abruptly form a bubble therein. With the increase in the volume of the bubble, the pressure of the liquid is increased so that a droplet of the liquid is ejected through an outlet communicating with the liquid channel to fly and attach to an ink supporting member, thus performing printing. Specifically, this liquid-jet printing head can easily be designed so as to have a multiplicity of nozzles provided at high density. Therefore, an increase in printing speed and an improvement in image quality can be achieved by adopting a lengthwise head arrangement.
In general, a liquid-jet printing head utilizing thermal energy to form a flying droplet of a printing liquid has a printing liquid heating means having a thermal energy transducer including a heating resistor element (hereinafter referred to as "heater") capable of developing heat to heat the printing liquid when an electric signal is applied to the element, and electrodes for applying the electric signal to the heating resistor element.
A printing liquid generally used for printing with liquid-jet printing apparatuses is a water-based printing liquid formed of printing components, such as pigments or dyestuffs, and a solvent component which is water or a mixture of water and a water-soluble organic solvent and in which the printing components are dissolved or dispersed.
A heating limit temperature at which such a water-based printing liquid is abruptly evaporated, i.e., a temperature at which vapor is generated at a gas-liquid interface by heat conducted through a very thin and stable vapor film formed between the electric heating surface and the liquid, is 250.degree. to 350.degree. C. Therefore, if a printing liquid having such a temperature characteristic is used for printing on a printing member in such a manner that an electric signal is applied to a heater to form a bubble in the printing liquid such that a flying droplet of the printing liquid is formed, the heater is heated from an ambient temperature to a temperature of 300.degree. to 800.degree. C. each time the electric signal is applied.
Recently, there has been a demand for a method or means for efficiently utilizing energy of a bubble formed as described above in order to further increase the liquid passage density and the number of liquid outlets. Japanese Patent Laid-Open Publication No. 4-211950 discloses a liquid passage structure in which the width of each of the liquid passages in the direction of arrangement of the liquid passages in a printing head has a maximum value between the corresponding outlet and an end of a thermal energy transducing element on the side of a supply port, and is monotonically reduced from the vicinity of the maximum value toward each of the outlet and the supply port through a certain passage portion, the reduction rate on the nozzle side being higher.
However, even if energy of a bubble can be efficiently utilized to eject of a printing liquid, the life of a printing head and the qualities of printed images are influenced by a cavitation collapse pressure which acts to erode a heater or a channel portion around the heater when a bubble caused by heating the printing liquid collapses, as long as the printing head is constructed to heat the printing liquid by repeatingly energizing the heater at a high temperature by electrical signals so that bubbles are formed in the printing liquid to eject droplets of the printing liquid. Therefore, design efforts have been made to reduce such an erosion effect.
For example, Japanese Patent Laid-Open Publication No. 59-138460 discloses a method of moving the position at which a bubble collapses out of the area of a heater to prevent erosion caused by cavitation collapse pressure by prescribing a certain positional relationship between the heater and a liquid channel portion which is formed in the vicinity of the heater so that the channel width is not constant.
Japanese Patent Publication No. 3-34467 discloses a liquid channel structure in which a rotational motion of a printing liquid is caused when the printing liquid flows into a liquid channel. Cavitation collapse pressure energy is consumed by the thermal motion to prevent heaters from being eroded by the cavitation collapse pressure.
Japanese Patent Laid-Open Publication No. 4-250051 discloses a method of providing a fluid resistance portion for minimizing the sectional area of a printing liquid channel wall upstream of a thermal energy transducing element. A bubble is thereby divided so that the energy of the bubble when the bubble collapses is reduced, thereby preventing erosion of the heater caused by cavitation collapse pressure.
Further, Japanese Patent Publication No. 4-292949 discloses a structure in which a passage surface located upstream of a thermal energy transducing element and facing a substrate portion on which the thermal energy transducing element is provided has a surface configuration such as to be formed of, from the upstream side, a first surface formed so as to monotonically reduce the sectional area of the printing liquid channel, and a second surface formed so as to monotonically increase the sectional area. In this structure, the position at which a bubble collapses, i.e., the position at which the maximum cavitation collapse pressure acts on a heater is separated from the heater surface, thereby preventing erosion of the heater caused by cavitation collapse pressure.
In the case of the art of Japanese Patent Laid-Open Publication No. 59-138460, erosion of a heater can be prevented, as described above, but the problem of breakage of a substrate, electrodes and other portions due to concentration of erosion action is encountered, since no means has been provided to prevent concentration of cavitation collapse pressure at portions other than the heater.
In the case of the art of Japanese Patent Laid-Open Publication No. 4-250051, and Japanese Patent Publication Nos. 3-34467 and 4-292949, the erosion action of cavitation collapse pressure upon a heater can be reduced or eliminated, but deposits from the liquid, generated by cycles of high-temperature heating of the heater to attach to the heater surface, cannot be easily separated from the heater surface.
If such deposits advances are not removed and if the deposition advances, the transmission of energy from the heater to the liquid becomes unstable and, accordingly, liquid ejection characteristics become unstable, resulting in a disturbance in image formation. Thus, there is a risk of a considerable reduction in the life of the head. The influence of deposits on the heater surface becomes greater if the size of the heater is reduced in order to achieve a high-density liquid passage arrangement.